I track my OG 12 answers in a spreadsheet. Most nights I'll pop a sleeping pill a couple of hours before bedtime (since they take about that long to kick in) and I'll work OG questions, keeping a moving average of my percentage correct. I noticed that about an hour after I take a sleeping pill, my moving average declines significantly. On the whole, I have been getting about 80%-90% right(depending on the section), but when I become sleepy that figure drops down to the 60%-70% level. I don't think it's from progressively harder questions, because when I resume the next day, my performance pops back up.

So don't underestimate a good night's sleep before the test!